the long-term objectives of the proposed research are to develop reliable and valid methods for studying the psychophysics of cardiac sensation in intact humans. For reasons developed in the proposal it is hypothesized that heart beat (HB) sensations are transduced through somatosensory mechanoreceptors and processed by perceptual mechanisms that are not specific to cardioception. To the extent that these hypotheses are true, individual differences in both somatosensory thresholds and in general perceptual processes should account for variations in the ability to detect HB sensations, an issue that has thus far eluded understanding. The project may help to interpret symptom reports by cardiac patients: e.g. why only some patients who have cardiac arrhythmias report palpitations. The proposed research has three specific goals: (i) To identify the sensory channels responsible for transducing HB sensations. This aim will be pursued by determining whether somatosensory thresholds to mechanical stimulation, for the Pacinian and/or non-Pacinian channels, are elevated by the temporal and/or spatial proximity of internal HB stimuli to the external mechanical stimuli. (ii) To identify the cardiac stimuli responsible for the HB sensation by examining the effects of experimentally-produced variations in myocardial performance on how accurately HB sensations are detected and their temporal location in the cardiac cycle. This goal requires identifying the cardiac performance parameters that best predict HB detection performance. Candidates to be investigated include stroke volume, systolic time intervals, and indices of left ventricular contractility. They will be monitored by impedance cardiography and manipulated by passive body tilt, static exercise, and dynamic exercise. (iii) To examine the role of attentional/perceptual processes in HB detection, by examining the effects of directed attention, number of stimulus presentations, and knowledge of results on the temporal location of HB sensations and the accuracy of their detection.